1. Field of the Invention
This invention pertains to the general field of multi-grounded neutral (MGN) electrical isolation circuits. In particular, it provides ground isolation between personal-communication-system (PCS) cell sites located on high-voltage transmission structures and the multi-grounded neutral system of a local electric utility's secondary low-voltage power service energizing the sites.
2. Description of the Prior Art
PCS cell sites typically require an antenna reaching 15 to 45 meters high from ground level and regulatory impediments often prevent the erection of support structures for such antennae in populated areas. Therefore, PCS cell sites are typically placed adjacent to high-voltage transmission towers which provide an existing, readily available structure for such antenna requirements. Power from the local electric utility's secondary low-voltage power service is then routed to the cell site to energize its operating components.
As in the case of all equipment connected to a utility power grid, current PCS cell-site state of the art practice is to achieve a low cell-site ground resistance by connecting it to the multi-grounded neutral of the public electric utility system. Typically such multi-grounded neutral is connected extensively to earth and is characterized by low impedance to remote earth, such as 2-5 Ohms.
As illustrated in FIG. 1, usually power is supplied from a medium-voltage transmission structure 10 to PCS cell-site equipment 12 through a transformer 14. This transformer reduces the voltage available from the network to the level required by consumers and feeds the cell site through a meter 16 and service-entrance equipment 18. As mentioned, the antenna of the cell site (not shown in the drawings) is commonly mounted on a high-voltage transmission tower 20 for convenience or necessity. Because of the resulting unavoidable proximity of the cell site 12 to the tower 20, the two are connected to ground through a common ground line 22. In addition, as illustrated by the connections shown in the diagram of the service-entrance equipment 18, the neutral lines 24 of the network are also connected to the same ground line. Thus, a common ground exist between the high-voltage transmission tower, the PCS cell-site equipment neutral, the transformer neutral, and the service entrance neutral. That is, a common ground exists for the whole system of power transmission and use.
Because of this grounding configuration, as is clearly understood by one skilled in the art, a ground potential rise (GPR) experienced at the site of the tower 20 as a result of a power system fault or lightening strike would be transferred directly to the consumers' grounding systems through the common neutral network. Thus, the current practice of connecting cell sites adjacent to high-voltage towers to the utility distribution system with a common multi-grounded neutral extends the zone of influence of a ground potential rise occurring at any such tower for a considerable distance along the power neutral that feeds the PCS cell site. This can cause severe damage to equipment and cables, and possibly represent a hazard for electrocution and/or ignition for any interconnected public utility customers. Additionally, because the multi-grounded neutral can carry a significant fraction of the fault current, it can induce detrimental common-mode voltages in any nearby conductor.
The main threat to the public is through the multi-grounded neutral interconnection. The source of the problem is the possibility that a large GPR can occur as a result of a power fault on the high-voltage transmission line. The magnitude of GPR and the corresponding voltage gradient established around a high-voltage transmission structure depends primarily on the grounding resistance of the transmission structure and the electrical impedance of the overhead-grounding conductors that are usually installed on these high voltage system. In a worst case scenario, substantially all voltage rise produced by a fault at a transmission tower could be transmitted to ground at the tower and propagated throughout the distribution network by the multi-grounded neutral line.
Therefore, there exists a critical need for isolating the neutral and ground lines of PCS cell sites located adjacent to high-voltage transmission towers from the neutral and ground of the electrical power grid servicing the site. This invention is directed at fulfilling this need.